My terrain become flat after I modify the unity’s terrain heightmap resolution, I notice the words “Please note that modifying the resolution will clear the heightmap, detail map or splat map” after I made the mistake 
Too bad…
Any solution? Thanks in advance 
You should’ve pressed Ctrl+Z to bring it back. Don’t know if it’ll work now.
Too bad it didn’t work From what I know, terrain is an asset, after modify, cannot be undo.
It can be undone, I have done it many times.
That sucks. I always take backups of my project every week or two just so I can recover from things like this. =(